The conversion of biomass into biofuel has taken on great significance in recent years as consumers and producers alike recognise the environmental and sustainability issues surrounding existing fossil fuels. The bulk of existing biofuel is derived from the fermentation of sugar crops and crops having high starch content, which will hereinafter be referred to as the “first generation” process. First generation processes typically involve an initial hydration step of mixing ground starch-based feedstock with water to form a slurry. The water may be pre-heated prior to being mixed with the feedstock. The slurry may additionally be heated in a vessel in order to activate the starch, and is then heated again and mixed with a liquefaction enzyme in order to convert the starch to long chain sugars. The activation stage typically uses steam-jacketed tanks or steam sparge heating to heat the slurry to the desired temperature. At the same time, agitation mixers, slurry recirculation loops, or a combination of the two mix the slurry. However, despite the presence of the recirculation pumps these heating methods can result in regions being created in the slurry tank or vessel whose temperature is much greater than the remainder of the tank. In such processes, starch hydrated early in the process can be damaged, e.g., denatured, if it comes into contact with these high temperature regions, resulting in a lower yield. These arrangements also do not provide particularly efficient mixing, as evidenced by the heat damage problem discussed above and also poor hydration of the starch.
These first generation processes normally use separate vessels for the activation and conversion stages of the process. Transfer of the slurry from the activation vessel to the conversion stage vessel is normally accomplished using centrifugal pumps, which impart a high shear force on the slurry and cause further damage to the hydrated starch as a result.
The conversion stage may also use steam- or water-jacketed tanks, or tanks heated by sparge heaters, to raise the temperature of the slurry to the appropriate level for the optimum performance of the liquefaction enzyme. Alternatively, jet cookers are employed to heat the incoming slurry into the conversion stage vessel. Not only can the slurry suffer the same heat damage as in the activation stage, but the high temperature regions also contribute to limiting the glucose yield from the process. The excessive heat of these regions promotes Maillard reactions, where the sugar molecules are destroyed due to interaction with proteins also present in the slurry. The combination of these Maillard losses with the shear losses from the transfer pumps limits the glucose yield available. Additionally, existing liquefaction processes require a long residence time for the slurry in the conversion stage to ensure that as much starch is converted to sugar as possible. This has a negative impact on the time and cost of the production process.
Crops with a high starch content have a high value in food applications (both in human and animal feed) and their sugar yield per hectare is low when compared to the potential sugar yield from cellulose and hemi-cellulose crops due to only a small percentage of the total crop being starch. Thus, a process for the derivation of biofuel from alternative sources of biomass, such as lignocellulosic biomass composed primarily of lignin, hemi-cellulose and cellulose, is of great significance to producers because lignocellulosic biomass is an extremely abundant biomass. It includes, e.g., all trees and grasses, as well as agricultural residues such as wet and dry distiller's grains, corn fibre, corn cob and sugarcane bagasse.
The process of deriving biofuel from lignocellulosic biomass will be hereinafter referred to as a “second generation” process. The second generation process converts the lignocellulosic biomass into alcohol (e.g. ethanol) in three stages: a first pre-treatment stage to disrupt the cellular structure of the biomass, a second hydrolysis stage in which the cellulosic part of the biomass is converted to short-chain sugars, and a third fermentation stage in which these sugars are converted to alcohol.
To increase the yield of the hydrolysis, the pre-treatment step is needed to soften the biomass and disrupt its cellular structure, thereby exposing more cellulose and hemi-cellulose material. Disruptive pre-treatment processes are normally chemical or physical in nature. Current chemical pre-treatment processes rely on a catalyst to achieve the desired disruption of the cells of the biomass. This catalyst is commonly an acid or an enzyme. The acid has the disadvantage of being harmful to the environment, whilst enzymes are relatively expensive. The most common physical pre-treatment process is steam explosion, examples of which are disclosed in Neves, U.S. Pat. No. 4,425,433 issued Jan. 10, 1984 and Foody, U.S. Pat. No. 4,461,648 issued Jul. 24, 1984. In steam explosion, the biomass is heated using high pressure steam for a few minutes, before the reactions are stopped by a sudden decompression to atmospheric pressure. A disadvantage of steam explosion is that the process must be contained within a suitable process vessel, and is thus a non-continuous process. Furthermore, the sugar yields from steam explosion are comparatively low while current costs for the process are high.
In both the first and second generation processes, yeast is used to ferment the sugars. However, the yeast is temperature sensitive and the biomass must be cooled to around 30° C. before the yeast can ferment the sugars. Cooling the biomass not only increases the length of the fermentation process, but also increases energy consumption given that the fermented biomass must be re-heated downstream for distillation.
The first generation process described above is the one most commonly used in the biofuel industry at present. In order to reduce the costs of transporting the crops for processing, biofuel processing plants are typically located in close proximity to the areas in which the crops are grown, or in areas with local markets for the two products from the process (e.g. ethanol and animal feed). In an effort to reduce costs still further, the starch-based components of the crop (e.g. corn kernels) are separated from the remainder of the crop (e.g. stalks and leaves) during harvesting, so that only the starch-based components are transported to the processing plant. However, in spite of this separation during harvesting around 10% by weight of the crop transported for processing is made up of lignocellulosic material (e.g. corn husks, corn cob) in which no starch is present. Thus, there is a negligible yield from 10% of the transported crop in a first generation process, even though that 10% is being transported to the processing plant.
A solution to this problem would be to also obtain alcohol from the lignocellulosic material present using the second generation process. However, having both first and second generation processes running alongside one another in a single processing plant has a significant impact on processing costs. Firstly, the set-up costs involved in constructing a processing plant having separate processing lines for the first and second generation processes will be much larger than that for constructing a plant with only a first generation process line. Secondly, the production costs in running the various stages of the two processes alongside one another will also be greater than those associated with running only a first generation process line.
Accordingly, one object of the present invention is to overcome one or more of the aforementioned disadvantages.